This invention relates generally to a continuously variable transmission (CVT) adapted for use in the driveline of an automotive vehicle. More particularly, it relates to such a CVT in which elements having fixed ratios are combined with a variator having a ratio which is continuously variable within predetermined limits. The CVT provides dual drive paths through the variator, with engine braking available at all times.
The typical automotive transmission is shifted in discrete steps between a high-torque, low-speed mode for starting a vehicle and a high-speed, low-torque mode for vehicle operation at highway speeds. In a manual transmission shifting is accomplished by the engagement of gear sets. In an automatic transmission shifting is accomplished by the controlled engagement of friction elements. Because such shifting is in step functions, the most efficient vehicle operation can only be approximated. Automotive engineers have long recognized that efficiency would be improved if the transmission could be adjusted continuously to compensate for changing loads, speeds, etc. This would allow the engine to be operated at maximum efficiency under changing conditions.
CVTs have been known for some time. The typical CVT employs a variator comprising a variable pulley with a pair of flanges mounted on a primary shaft such that one of the flanges is movable axially with respect to the other. Another, similar variable pulley is mounted on a secondary shaft. A flexible belt couples the pulleys for transferring torque therebetween when the primary shaft is driven. When the pitch radius of one pulley is changed, the pitch radius of the other pulley is changed simultaneously in the opposite direction. As a result, the drive ratio between the shafts is variable in a continuous, smooth manner within the ratio range of the variator. Typically, the limits of this range are an underdrive or reduction ratio and an overdrive ratio.
In recent years considerable work has been directed to the application of a CVT to the driveline of an automotive vehicle. In an automotive environment in particular, space constraints often dictate housing configurations, shaft center distances, pulley diameters, belt sizes, etc. Design parameters of this nature limit the ratio range and/or torque capacity of a CVT.
An important step in the development of a CVT suitable for use in an automotive vehicle is disclosed in copending U.S. application Ser. No. 564,856 filed Dec. 23, 1983. That application is assigned to the assignee of this application, and is incorporated herein by reference. Briefly, it discloses a dual-pass CVT including an input, an output, and a variator. Gear sets and one-way clutches establish a first drive path from the input through the variator in one direction to the output. Other gear sets and jaw clutches establish a second drive path from the input through the variator in the opposite direction to the output. Initially torque is transferred through the first drive path as the ratio of the variator is varied from maximum underdrive to maximum overdrive. At that point the variator ratio is equal to the corresponding gear ratios, and the system is in substantial synchronism. The jaw clutches are engaged, and the one-way clutches disengage when torque subsequently is transferred through the second drive path as the ratio of the variator is varied in the opposite direction.
When that CVT is in the high-range mode; that is, when it is configured for the second drive path, the jaw clutches are positively engaged, allowing the CVT to provide engine braking. However, when the CVT is in the low-range mode; that is, when its configuration is for the first drive path, the CVT does not provide engine braking. Rather, the one-way clutches disengage when the direction of torque transfer is from the vehicle wheels through the CVT to the engine. This may occur, for example, when the vehicle operator closes the throttle, allowing the engine to run at idle speed. In this event the typical transmission control system would upshift the transmission in the interest of economy. For a dual-pass CVT, the typical control system would configure it for the second drive path. The process of so configuring a dual-pass CVT could take several seconds, during which time no engine braking would be available. Thus there is a need in the art for a dual-pass CVT which provides engine braking during this transition period, so that engine braking is available at all times.